Today could mark the conclusion of Microsoft Corp.’s marathon antitrust case, as the last plaintiff standing – Massachusetts Attorney General Tom Reilly – makes his case before the D.C. Circuit U.S. Court of Appeals.

The odds aren’t on Massachusetts’ side, legal experts say, as the Justice Department and 20 states already have settled with the software giant.

“Bit by bit, Microsoft has been resolving these cases,” said Mark Ostrav, an antitrust expert with Mountain View, Calif., law firm Fenwick & West. “They’ve been getting off cheap on all these deals.”

Once considered a threat to Microsoft’s very structure, the antitrust case has dwindled to little more than a nuisance – “pocket change for Bill Gates,” Ostrav said.

The Massachusetts appeal stems from a settlement approved by U.S. District Judge Colleen Kollar-Kotelly in 2001. After being found guilty of illegally maintaining a monopoly with its Windows operating system, Microsoft was ordered to license its computer code and allow computer makers to feature rival software on the desktop.

Nine states asked Kollar-Kotelly for stricter sanctions, but eight eventually settled with Microsoft – a development Ostrav blames on taxpayer fatigue.

Many Microsoft critics have written off the U.S. challenge, as even an outstanding antitrust case from Sun Microsystems doesn’t look likely to change the company’s practices too drastically.

The real fight will begin this month across the Atlantic, Ostrav said, as Microsoft gears up for a similar battle with European regulators. There the case favors the prosecution, and not the defense.